Carbon materials are base materials used in various applications in view of high chemical stability and high heat resistance, high conductivity and high strength, high workability, high biocompatibility and the like. Conventional carbon materials include highly conductive carbon black, carbon nanotube, fullerene, and graphene. However, since they are in the form of particles and the particles are not joined with each other, for example, even when they are composited with a resin as fillers, the effect of improving the strength is limited. Also, since infiltration of the resin into voids in the filler is limited so that the voids remain in the filler, the effect of improving the strength is limited in the case of forming the composite material.
Also, in particular, carbon fiber which is an example of carbon materials other than those described above is used in various applications with a focus on a structural material in view of the characteristics, for example, strength, elastic modulus, chemical and thermal stability, high conductivity, light specific gravity when compared with metal and the like. Further, in the case of using as the structural material, it is often used by compositing with a thermosetting or thermoplastic resin. However, affinity between the surface of carbon fiber and the resin is low and as to the decrease in the strength of the composite material due to peeling, various investigations have been made with a focus on surface treatment of the carbon fiber. For example, a method for porosifying a surface of carbon fiber by an activation treatment is described in Patent Document 1.
However, only by the surface treatment of carbon fiber having a diameter of approximately several μm, a surface which can contribute to the improvement of adhesive force is only present on the surface of carbon fiber subjected to the treatment and a specific surface area is actually small, and therefore, an interface contacting the resin with the carbon fiber is small and there is a limit to the improvement in peeling strength. Therefore, the improvement in peeling strength has been investigated by exerting an anchoring effect of the resin in addition to by porosifying not only the surface but also the whole of carbon fiber to increase the specific surface, thereby increasing the contact interface between the resin and the carbon fiber.
For example, an example of introducing a continuous porous structure into a carbon material itself by mixing a thermosetting resin with a thermoplastic resin, curing the thermosetting resin, and then performing carbonization after removing the thermoplastic resin is shown in Patent Document 2. Also, a method for obtaining a porous carbon fiber by spinning a combination of incompatible polymers with each other, followed by stretching is disclosed in Patent Document 3.